1. Field of the Invention
The present invention relates to means and method for providing augmented, enhanced contrast of electron video signals of alignment marks with respect to microcircuit substrates and coatings thereon.
2. Description of the Prior Art
Direct wafer electron beam lithography requires re-registration of patterned fields to substrate fields. For very high resolution devices and circuits with line widths as narrow as 0.5 microns, the re-registration must be accurate to better than 0.1 microns. This level of precision demands very accurate scanning electron microscopy detection of the alignment marks, generally as a line scan, and fast positional control feedback by means of a digital computer. In the past, such registration has been based on topographical changes which modulate the secondary electron emission which, when directly viewed without layers thereon, produces acceptable contrast of the video signals. However, upon the application of a layer of resist, for example, poor contrast of the video signal results because the resist tends to smooth the sharp steps or edges of the underlying surface, such as formed by an oxide cut. Therefore, the video signal-to-noise ratio from secondary electrons is insufficiently large to provide the required video signal contrast.
As taught in the above-referenced copending patent application Ser. No. 601,944, which overcomes these and other problems, by utilizing high atomic number metals or compounds as electron beam registration aligmnment marks on low atomic number substrates, improved secondary and backscattered electron video signals are obtained.